Shakespeare's pronunciation

Elizabethan pronunciation differed significantly from our
own. Vowels were in the process of changing, in a process
known as a "vowel shift"--the same process that has given us
so many different accents today. Thus there are a number of
words that would have made perfect rhymes that now sound like
half-rhymes: "love" and "prove," for example.

In Henry IV, Part One, Falstaff tells Hal, seemingly
inexplicably, "If reasons were as plentiful as blackberries,
I would give no man a reason upon compulsion." There is a pun
here, but the modern audience would be hard-pressed to notice
it, unless "reason" were pronounced in the Elizabethan
manner, which would sound something like "raisin." The pun
then becomes obvious, and the line makes much more sense.
(Click to listen* to this passage.)

In Julius Caesar, Cassius puns on "Rome" and
"room"-- and again the words were pronounced alike. (Click to
listen* to this passage.)

Footnotes

Of raisins and reasons

Poins: Come, your reason, Jack, your reason.Falstaff: What, upon compulsion? Zounds, and I
were at the strappado or all the racks in the world, I
would not tell you upon compulsion. Give you a reason on
compulsion? If reasons were as plentiful
as blackberries, I would give no man a
reason upon compulsion, I.
(Henry IV, Part One, 2.4.246-42)

Zounds

A contraction of "by God's wounds."

Strappado

The victims were tied by straps on the hands, suspended,
and dropped varying heights until they confessed to
whatever was required.

Of room and Rome

The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings.
. . . . . . . . . . .
When could they say (till now) that talked of Rome,
That her wide walks encompassed but one man?
Now is it Rome indeed, and
room enough,
When there is in it but one only man.
(Julius Caesar, 1.2.140-41, 154-57)